In a known type of reactor, for example, as used in the Dresden Nuclear Power Station near Chicago, Ill., the reactor core is of the heterogeneous type. That is, the nuclear fuel is in the form of elongated cladded rods. These fuel rods or elements are grouped together and contained in open-ended tubular flow channels to form separately removable fuel assemblies or bundles. A sufficient number of fuel assemblies are arranged in a matrix to form the nuclear reactor core capable of self-sustained fission reaction. The core is submersed in a fluid, such as light water, which serves both as a coolant and as a neutron moderator.
A typical fuel assembly may be formed, for example, by an 8 .times. 8 array of spaced fuel rods supported between upper and lower tie plates, the rods being several feet in length, approximately one-half inch in diameter, and spaced from one another by a fraction of an inch. To provide proper coolant flow past the fuel rods it is important to maintain the fuel rods in fixed spaced relation and restrain them from bending and vibrating during reactor operation. A plurality of fuel rod spacers positioned in spaced relation along the length of the fuel assembly are provided for this purpose. Such fuel rod spacers are shown, for example, by J. L. Lass et al in U.S. Pat. No. 3,654,077.
A problem in the design of such a fuel assembly is to provide an efficient, effective arrangement to maintain the fuel rod spacers in their proper axially spaced positions without the use of excessive structural materials. It is extremely important to minimize the amount of structural material in the core of a reactor because such material is a neutron poison which unproductively captures neutrons and an additional amount of costly fuel may be required to compensate for the resulting neutron loss.
Several approaches to the problem of retaining fuel rod spacers in their proper axial positions are known. In an arrangement shown in U.S. Pat. No. 3,379,618, the fuel rod spacers are attached to a plurality of struts which span the length of the fuel assembly and are secured between upper and lower tie plates. An arrangement shown in U.S. Pat. No. 3,375,172 is somewhat similar except that the struts or longitudinal support members are secured to only one of the tie plates whereby their length, and hence the amount of strut material, may be somewhat reduced.
In another known arrangement the fuel rod spacers are secured in axial position by one or more segmented fuel rods. For example in one such arrangement the connectors between the individual segments of the segmented fuel rods are formed with flanges which engage and secure the fuel rod spacers. Such a segmented fuel rod spacer capture arrangement is shown, for example, by J. L. Lass in U.S. Pat. No. 3,466,226. While the segmented rod arrangement reduces the amount of structure material in the core, as compared to the other arrangements mentioned above, the manufacture of the segmented rod is complicated and expensive. Also the discontinuities in the fuel column caused by the segment connectors are undesirable.
Another arrangement for axially positioning the fuel rod spacers is illustrated in U.S. Pat. No. 3,802,995. There, axially spaced lugs are provided on a water tube with each lug fitting between spaced elements forming the fuel rod spacer. Such arrangement overcomes many of the shortcomings expressed above with respect to arrangements prior thereto; however, even this arrangement necessitates relatively expensive square plug or extension fitting into a relatively expensive square hole in the bottom tie plate to prevent the tube from rotating and disengaging the lugs from the spacers. Moreover, this arrangement does not prevent the insertion of the fuel rod spacer into the assembly upside down.
Accordingly, it is an object of the present invention to provide an improved nuclear fuel element spacer retaining means which obviates the relatively expensive square end plug and also provides assurance that the fuel rod spacer will not be installed into the assembly upside down.